Pneumatic rubber passenger and truck tires are composed of elements which conventionally include a tread of a rubber composition. The tread rubber is sometimes desirably compounded to provide a tire with a relatively low rolling resistance with reasonable wear and traction.
Viscoelastic properties of the tread rubber itself are important and basically determinative considerations for its performance, particularly for rolling resistance and skid resistance of the tire.
Although it may be desired to compound the tire's tread composition to reduce the rolling resistance of the tire without substantially degrading the tire's traction features, the tire's traction might be expected to be somewhat sacrificed as may be evidenced by its reduction in wet and dry skid resistance.
Tire treads are often composed of synthetic rubber or blends of synthetic rubber with natural rubber for the purpose of achieving desirable tire tread characteristics such as wear, traction and reduction in rolling resistance. Various synthetic rubbers have been used in the manufacture of tires with such treads including styrene/butadiene copolymers (prepared by emulsion or solution polymerization methods) sometimes referred to as SBR, high cis 1,4-polybutadiene rubber as well as medium and high vinyl (1,2-) polybutadiene rubbers. Sometimes a synthetic cis 1,4-polyisoprene has, at least in part, been substituted for the natural rubber in tire tread compositions.
Although such rubber compositions are taught to provide various benefits, some for tire treads it continues to be desirable to provide a pneumatic tire having a rubber tread having an enhanced rolling resistance and/or treadwear commensurate with reasonable traction qualities.
Viscoelastic properties of the rubber, or rubber blend, itself are important. For low rolling resistance of the tire, a tan. delta optimization for a temperature in the range of about 40.degree. C. to about 60.degree. C. is desired whereas a tan. delta optimization for a temperature range of about -10.degree. C. to about 20.degree. C. is desired for good skid resistance. It is difficult to adjust a rubber blend for a tan. delta optimization substantially simultaneously for both temperature ranges and, thus, for both rolling resistance and skid resistance.
In one aspect, U.S. Pat. No. 4,894,420 discloses a tire with a tread composed of (A) 30-90 parts cis 1,4-polyisoprene rubber; (B) 5-20 parts isoprene/acrylonitrile and/or butadiene/acrylonitrile copolymer rubber; and (C) 0-35 parts of other rubber which can be cis 1,4-polybutadiene rubber. Skid resistance is taught to be enhanced without sacrifice in rolling resistance.